A control system is, typically, a closed-loop system that depends upon the feedback concept for operation. Conventiona11y, the output or controlled parameter is forced to be a function of the reference input, where the reference input is, in general, an arbitrary function of time.
One example of a control system is the conventional automatic throttle control or, more simply, the autothrottle. Essentially, an autothrottle provides automatic control of the throttles of, for example, an aircraft and, in this application, is employed, for example, during climb, descent, cruise, and landing approaches to significantly reduce pilot workload. Additionally, because, at high altitudes, throttle settings are the most important consideration in fuel savings, an autothrottle can reduce fuel consumption and thus increase the range of the aircraft.
Customarily, the autothrottle utilizes airspeed as the basic control parameter: although other parameters, such as rate-of-climb or angle of attack, can be used. Known autothrottles typically comprise a main controller which provides an airspeed command which is compared with the actual airspeed measured by the airspeed indicator. Using the feedback concept, an airspeed error signal is generated and provided back to the controller. Also inputted to the controller are signals indicative of pitch attitude as from a gyroscope, longitudinal acceleration from an accelerometer, and airspeed rate as from an airdata sensor. Based on these inputs, the controller generates a command signal which is provided to an electronically controlled A-C motor operated as a position servo. This servo is connected in parallel with the throttle levers and engine controls through a drum type friction clutch pack. Using feedback control, the throttle levers are set in accordance with the command signal.
Because the typical autothrottle is installed at the time of construction of the aircraft, the autothrottle generally includes a centrally located console from which the pilot selects the desired throttle modes, accomplishes throttle servo engagement/disengagement, and inserts airspeed commands into the system. Location of these functions on the aircraft's control wheel provides a degree of pilot relief and is the standard configuration.
While operationally quite efficient, known autothrottle systems do not lend themselves to application to an aircraft after the construction thereof has been completed. Considerable modification to the aircraft is required to apply the autothrottle components which require a considerable amount of time and effort and results in an increased cost of the overall system. Additionally, the feedback concept employed by known autothrottle systems increases the opportunity for signal errors which can result in the improper functioning of the system.